This invention provides improvements to the current processes of extracting soda ash from trona ore. Trona ore is found in massive, deep rock trona deposits in sites such as Green River, Wyoming or Beypazari, Turkey. A typical profile of the trona deposit found in Green River, Wyoming is as follows:
______________________________________ CONSTITUENT PERCENTAGE ______________________________________ Sodium Sesquicarbonate 90.00% NaCl 0.1 Na.sub.2 SO.sub.4 0.02 Organic Matter 0.3 Insolubles 9.58 Total 100.0 ______________________________________
The main constituent of trona ore is sodium sesquicarbonate. Because of the presence of other constituents, trona ore needs to be processed to remove them in order to produce soda ash (Na.sub.2 CO.sub.3) or other sodium-containing chemicals like sodium bicarbonate (NaCO.sub.3) or caustic soda (NaOH). The prior art provides many methods to recover soda ash and other values from trona ore. These extraction processes can be classified as either dry mined or solution mined methods. Each type of mining is well known to persons of ordinary skill in the art as giving rise to trona ore. The trona may be solid or in the form of a solution, slurry, or suspension in the case of solution mining; all may be employed in connection with the present invention. Nahcolite ore has also been used for such purposes, albeit to a lesser extent. The present invention is applicable to nahcolite ores as well as to trona, however trona ore processing will be discussed throughout.
Two major types of processes using dry mined trona are typical and are practiced, e.g., in Green River, Wyo. The monohydrate process for obtaining soda ash from trona ore involves dry mining the ore. The ore is crushed into small pieces and then calcined through the application of heat to drive off water and carbon dioxide gases from the trona. The calcinate is then dissolved in water or dilute alkaline liquors which are recycled from the downstream process. The solution is then passed through settlers, clarifiers and/or thickeners, where insolubles are removed to waste. The process stream is then preferably sent through a carbon treatment and filtration step. The resulting liquor, essentially free of bicarbonate, is preferably sent to evaporative crystalizers to crystallize sodium carbonate monohydrate. This material can then be calcined to produce anhydrous soda ash product.
The sesquicarbonate process crushes trona ore prior to dissolution in recycled, diluted, mother liquor, followed by separation of insolubles such as via clarifiers and thickeners. Ore insolubles are comprised of oil shales and minerals such as dolomite and other minerals, which are not very soluble in the alkaline process liquors. The insolubles-free liquor, containing carbonate and bicarbonate, is fed to cooling crystallizers to crystallize sesquicarbonate which can then be calcined to soda ash.
A solution mined recovery process is described in U.S. Pat. No. 5,283,054, which is incorporated herein by reference. In the processes described therein, trona ore is solution mined using an aqueous solvent to form a solution known as "brine" which is high in total dissolved solids. A substantial proportion of the dissolved solids is made up of alkali. The alkali content (alkali value) is typically 12 to 17.5 weight percent sodium carbonate and about 3 to 5 weight percent sodium bicarbonate. Actual values of sodium carbonate and sodium bicarbonate can vary due to such factors as temperature and residence time of ore in contact with the mining solution. Recovery of alkali values is typically obtained through heating the brine at temperatures of about 100.degree. C. to about 140.degree. C. to convert sodium bicarbonate to sodium carbonate and drive off any resulting carbon dioxide. The partially stripped brine is concentrated, where additional CO.sub.2 may be removed. The resulting brine, which typically contains a reduced sodium bicarbonate content, is then preferably reacted with an aqueous sodium hydroxide solution (generally formed by causticizing sodium carbonate values with calcium oxide or calcium hydroxide) in such amounts as to convert most of the remaining sodium bicarbonate in the brine to sodium carbonate. The sodium hydroxide-treated brine is then preferably cooled to a temperature of about 25.degree. C. to 5.degree. C., precipitating sodium carbonate decahydrate crystals. The sodium carbonate decahydrate crystals are separated from their mother liquor through means such as a centrifuge. The separated decahydrate crystals can then be dissolved to form a sodium carbonate solution. This solution can be heated, such as to a temperature range of from about 60.degree. C. to approximately 110.degree. C., to evaporate water and crystallize sodium carbonate monohydrate crystals. The sodium carbonate monohydrate crystals can then be calcined to produce soda ash.
Nahcolite ore, comprised mostly of sodium bicarbonate, is another mineral which can serve as a source of soda ash.
An unfortunate and highly disadvantageous by-product of both the dry and solution mined processes is the formation of magnesium or other deposits or scale on the surfaces of the processing equipment and otherwise. In addition to the desired high alkali values, the process solution can also contain magnesium and other impurities, which can contribute to such scale. The magnesium values are especially problematic in a solution mined processes where feed liquor is obtained by solution mining, due to the long residence time of the aqueous solubilizing medium in the mine and/or the increased opportunity to acquire such values when more than one stratum is mined. Similarly, magnesium values can also be present after dry-mined ore is dissolved in an aqueous medium. In either dry mined or solution mined processes, the magnesium can precipitate out during processing, forming scale on the surfaces of heat exchangers and other processing equipment.
Some of the scale is a result of the precipitation of eitelite (Na.sub.2 Mg(CO.sub.3).sub.2). There are three sets of soda ash processing conditions which chiefly lead to eitelite scale formation.
These conditions are 1) Promoting CO.sub.2 removal from the liquor, converting at least a portion of the NaHCO.sub.3 to Na.sub.2 CO.sub.3, regardless of temperature, 2) increasing the liquor temperature, and 3) concentrating the magnesium in the solution mined feed, regardless of temperature or carbonate/bicarbonate concentration. When the equilibrium magnesium solubility is exceeded, scale can potentially form.
U.S. Pat. No. 5,283,054 describes a process whose first step is to preheat the liquor prior to CO.sub.2 stripping. In this process, most of the CO.sub.2 stripped liquor is sent to an evaporator in order to concentrate the liquor prior to crystallizing the intermediate carbonate decahydrate. Significant quantities of eitelite scale can form in the stripping column and evaporator preventing the mass transfer rates required to evolve CO.sub.2 from the solution. In addition, heat exchangers used to preheat the solution mined-based liquors can become scaled.
U.S. Pat. No. 5,262,134 describes a solution mine based process. After heating the mine liquor to remove some of the CO.sub.2 and evaporate water, the process stream is subjected to a cooling crystallization to remove sesquicarbonate. Eitelite scale formation can be a problem with this process as well.
U.S. Pat. No. 5,609,838 describes a solution mining related process which uses steam stripping at 90.degree. C. or below to partially decompose sodium bicarbonate as a first step in the process. Prior to feeding the CO.sub.2 stripper, the mine liquor is passed through a heat exchanger where scale is likely to form.
Solution mine-derived feeds can also be fortified with either calcined, or uncalcined, dry mined trona ore. After separating the ore insolubles via clarification or filtration, the liquors containing Na.sub.2 CO.sub.3, NaHCO.sub.3, and dissolved magnesium can give rise to scale formation if this equilibrium solubility is exceeded downstream in the process.
Exemplary flow diagrams and process descriptions for some useful processes are provided by Isonex Inc. at its Internet web site, www.isonex.com.
Elevated process temperatures present in the soda ash extraction process and the inverse solubility of magnesium salts like eitelite foster precipitation of magnesium as scale since Mg solubility decreases with increasing temperature. When the equilibrium solubility is exceeded, via heat exchange or otherwise, scale formation is likely to result.
When feed, especially from solution mined ore, is evaporated at elevated temperatures with CO.sub.2 stripping, scale formation is more likely to occur. The following tables set forth data comparing evaporation of typical process liquor at 104.degree. C. with and without CO.sub.2 stripping.
TABLE I ______________________________________ Evaporation Without CO.sub.2 Removal % % ppm % Bicarbonate % Mg Na.sub.2 CO.sub.3 NaHCO.sub.3 Mg Decomposed Precipitated ______________________________________ Feed Liquor 13.0 4.6 40 Concentrated 20.8 7.0 62 4 2 Liquor ______________________________________
TABLE II ______________________________________ Evaporation With CO.sub.2 Removal % % ppm % Bicarbonate % Mg Na.sub.2 CO.sub.3 NaHCO.sub.3 Mg Decomposed Precipitated ______________________________________ Feed Liquor 13.0 4.6 40 Concentrated 22.8 3.80 46 47 26 Liquor ______________________________________
Since the temperature was constant (at the atmospheric pressure boiling point) for both examples, it is clear that CO.sub.2 removal reduces the ability of magnesium to supersaturate and assists in its precipitation.
Equilibrium magnesium solubilities in solutions containing a range of sodium carbonate and bicarbonate concentrations also show that as the carbonate-to-bicarbonate ratio increases, the solubility of magnesium decreases. The following equilibrium magnesium solubilities were measured for reagent grade (pure) solutions which were also saturated in alkali content at a constant temperature of 20.degree. C.
TABLE III ______________________________________ Equilibrium Mg Solubilities % Na.sub.2 CO.sub.3 % NaHCO.sub.3 ppm Mg ______________________________________ 6.0 6.3 64 9.0 5.6 60 12.0 4.9 54 14.0 4.5 50 17.0 4.0 44 ______________________________________
When the equilibrium magnesium solubility is exceeded, conditions are ripe for eitelite precipitation and scale formation, regardless of temperature.
Increased magnesium concentration in the feed, regardless of temperature or carbonate/bicarbonate concentration, can also cause scale formation when the equilibrium magnesium solubility is exceeded. This is a particular problem in solution mined processes.
Scale formation generally increases as the process temperature increases. As a result, evaporator surfaces and process equipment that are elevated in temperature are particularly prone to scale. Scale retards the efficiency of the heat exchanger as the scale levels increase. Scale may also partially, or completely, close passageways or other openings in the processing system, and impede the flow of the process solution through the various stages of the soda ash recovery process. Scale is difficult to remove and can generally be done only through such inconvenient and expensive means as acid washing or acid cleaning. A main drawback to acid washing is that the processing line needs to be shut down in order to remove the scale. Shutting down the production line is an unacceptable or unattractive option to most soda ash producers. Delaying or increasing the intervals between acid washing or acid cleaning would be highly advantageous.
Methods for reduction of magnesium scale in soda ash production have been considered but are believed not to be as useful as the present processes. For example, one proposed solution was to lower the heat exchanger or evaporator skin temperatures to reduce the precipitation of magnesium. This, however, was only a partial solution because magnesium precipitation still occurred.
A number of chemical additive materials were tried in unsuccessful attempts to reduce or eliminate magnesium scale in soda ash recovery processes. Such materials included phosphates such as sodium tripolyphosphate, Na.sub.5 P.sub.3 O.sub.10, and tetrasodium pyrophosphate, Na.sub.4 P.sub.2 O.sub.7. These were found not to achieve acceptable reduction in scale formation. Glassy (amorphous) phosphates, having formula [NaPO.sub.3 ].sub.n. PO.sub.4, made by the FMC Corporation and sold under the following trade names were also employed without success: SODAPHOS (glassy (amorphous) phosphates) (n=6), HEXAPHOS (glassy (amorphous) phosphates (n=13), and (glassy (amorphous) phosphates) (n=21). Each of the foregoing phosphates was tested at 100 ppm of the compound in laboratory batch evaporation and CO.sub.2 stripping tests. Duplicate sets of tests were run, yielding essentially the same, unacceptable, results.
Commercially available water treatment products were also tried without success. BELSPERSE 161 (phosphinocarboxylic polymer) and Belclene 200.RTM. are aqueous water treatment products made by FMC Corporation's Process Additives Division. BELSPERSE 161 contains a phosphinocarboxylic polymer and the product literature indicates "good" Ca and Mg sequestration ability. It also functions as a dispersant to help prevent adherence to heat transfer surfaces. BELCLENE 200 (low molecular weight maleic acid polymer) is a low molecular weight maleic acid polymer which functions primarily by altering the crystal structure of mineral scale deposits.
Several antiscalant products sold by Betz Laboratories were also tried in two sets of tests; the first used 20 ppm of each: GCP 187 (antiscalant products sold by Betz Laboratories) and GCP 9319 (antiscalant products sold by Betz Laboratories). The chemical makeup of these products is not known.
Betz Laboratories water treatment products which contain alkylepoxycarboxylates (AEC), which are advertised in the trade literature as alternatives to phosphonates for inhibiting the formation of calcium carbonate scale, were also evaluated. It is believed that these products contain mixtures of chelating agents and threshold inhibitors. Products GCP 9313 (alkylepoxycarboxylates), GCP 9317 (alkylepoxycarboxylates), GCP 9318 (alkylepoxycarboxylates), and GCP 9322 (alkylepoxycarboxylates) were each tested at 25 ppm of the as-sold product. These were determined to be ineffective in controlling scale in alkaline, highly concentrated, soda ash process liquors.
Two Nalco Company products, NALCO 9721 (aqueous solution of an organic polymer salt) and NALCO 9762 (aqueous solution of an organic polymer salt) were also tried at 25 ppm and found to be ineffective as antiscalants. Additionally, certain DEQUEST brand products of the Monsanto Company were tried at 20 ppm as the as-sold product and were found to be ineffective. These were DEQUEST 2000 (phosphonate compounds), 2006 and 2060.
VERSENOL 120 (compound based upon the trisodium salt of N-hydroxyethylethylenediamine tricetic acid) is sold by Dow Chemical Corporation and is based upon the trisodium salt of N-hydroxyethylethylenediaminetriacetic acid. This chelating agent was also tried at 20 ppm on the as-supplied product basis without acceptable results.
Zeolite-A, an aluminosilicate molecular sieve, which functions by selectively absorbing cations and/or anions within its inorganic matrix, was also used. Selectivity is based upon the nature of its porous matrix, particularly pore size. This is commonly used in European laundry detergents to sequester calcium and magnesium- based "dirt." However, at suspended concentrations between 100 and 5,000 ppm, the zeolite was ineffective in controlling magnesium deposition from alkaline liquors in laboratory CO.sub.2 stripping and evaporation tests.
It is therefore greatly desired to provide means of reducing or inhibiting magnesium scale formation on the surfaces of heat exchangers or other processing equipment which can be easily applied to soda ash extraction processes without adding additional processing steps or effecting the yield or chemistry of the resulting soda ash or sodium carbonate.